I'm having problems with a design using the LM43603. I'm using several of them with different output voltage. But for this case I will just mention specs: Vout 3.3V, 1.0 A. Vin 10-33V
I'm using the recommended approach in the datasheet. and default switch frequency of 500 kHz The inductor is selected to reduce ripple current (datasheet recommend 20-40%). The selected inductor is 39 uH since I aim for a lower rpple current. However, the webbench suggest a much smaller inductor (5.2 uH). Clearly the ripple current exceeds datasheet max recommendations.
In webbench I have tried customizing the design and select L=39 uH. I also select Rfbt=220k and Rfbb=97.6k and adjust Cff accordingly. However, I'm unable to freely select Cout. It is limited to around 47 uF, but I have more. So in webbench I cannot simulate with the actual Cout and the value of Cff calculated using the equation in the datasheet.
Anyway, with the possible design changes, webbench claims the design i unstable. Phase margin becomes too small for low Vin.
Unfortunately I cannot simulate my exact design due to the limitations of webbench and the datasheet does not provide sufficient information to calculate phase margin myself.
Questions:
How is phasemargin calculated for the LM43603? What is the loop transfer function?
Is there a work-around to allow free selection of output capacitors for the Webbench?